title: Hormonal risk factors and invasive epithelial ovarian cancer risk by parity.
abstract: BACKGROUND
  Recent studies have suggested that several ovarian cancer risk factors differ by parity status, but these findings have not been confirmed. We evaluated whether known risk factors of ovarian cancer differ between nulliparous and parous women using data from two large prospective cohorts.
  
  
  METHODS
  Data from the National Institutes of Health-AARP Diet and Health Study and the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial were combined for this analysis. Cox regression models were used to estimate associations with ovarian cancer risk. Risk heterogeneity by parity status was assessed using likelihood-ratio tests.
  
  
  RESULTS
  Among the 125 437 women included in the analysis, there were 16 589 (13%) nulliparous women and 108 848 (87%) parous women. Of the 623 women diagnosed with invasive epithelial ovarian cancer, 102 (16%) were nulliparous and 521 (84%) were parous. While parity reduced ovarian cancer risk, no differences were found for other risk factors by parity. Among ever users of hormone therapy, body mass index suggestively increased the risk of ovarian cancer by 1.5-fold in nulliparous but not parous women (P-heterogeneity=0.08).
  
  
  CONCLUSIONS
  While nulliparous women have higher ovarian cancer risk than parous women, our findings suggest that the relative effects of most other risk factors do not differ by parity.
authors.affiliation: Division of Cancer Epidemiology and Genetics, National Cancer Institute, NIH, DHHS, Rockville, MD, USA. clara.bodelon@nih.gov
authors.forename: C
authors.initials: C
authors.lastname: Bodelon
authors.forename: N
authors.initials: N
authors.lastname: Wentzensen
authors.forename: S J
authors.initials: SJ
authors.lastname: Schonfeld
authors.forename: K
authors.initials: K
authors.lastname: Visvanathan
authors.forename: P
authors.initials: P
authors.lastname: Hartge
authors.forename: Y
authors.initials: Y
authors.lastname: Park
authors.forename: R M
authors.initials: RM
authors.lastname: Pfeiffer
mesh_terms: D000368:Aged; D000077216:Carcinoma, Ovarian Epithelial; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008875:Middle Aged; D009375:Neoplasms, Glandular and Epithelial / Q000453:epidemiology*; D010051:Ovarian Neoplasms / Q000453:epidemiology*; D010298:Parity; D011446:Prospective Studies; D012044:Regression Analysis; D012307:Risk Factors; D014481:United States / Q000453:epidemiology